Power supply unit for wire electrical discharge machining and method of wire electrical discharge machining

ABSTRACT

A power supply unit for wire electrical discharge machining using an ac high-frequency power supply for applying an ac high-frequency voltage between a wire electrode (1 a ) and a workpiece ( 2 ) includes a high-frequency oscillation and amplification circuit ( 4 ) and a pulse power supply ( 5 ) which is an ac-high-frequency-voltage intermittently supplying unit for effecting the application of the ac high-frequency voltage between the electrode and the workpiece and a pause thereof. In wire electrical discharge machining using this power supply unit for wire electrical discharge machining, it is possible to suppress the vibration of the wire electrode (1 a ) due tog the electrostatic force acting between the wire electrode (1 a ) and the workpiece ( 2 ), making it possible to improve the straightness accuracy of a machined surface of the workpiece ( 2 ) and make surface roughness small.

TECHNICAL FIELD

The present invention relates to improvements in a power supply unit forwire electrical discharge machining and a method of wire electricaldischarge machining which are used in wire electrical dischargemachining for machining a workpiece by generating electric discharge inan electrode gap between a wire electrode and the workpiece to supplymachining electric power to the electrode gap.

BACKGROUND ART

As a conventional power supply unit for wire electrical dischargemachining for supplying machining electric power between an electrodeand a workpiece, it is known that a fine machined surface can beobtained in the workpiece by applying an ac high-frequency voltagebetween the electrode and the workpiece and generating electricdischarge of short time durations at a high frequency of repetition. Forexample, it is disclosed in JP-A-61-260915 that a machined surface of 1μm Rmax or less can be obtained by applying an ac high-frequency voltageof 1 MHz to 5 MHz between the electrode and the workpiece. In addition,it is disclosed in JP-A-7-9258 that a machined surface of 0.5 μm Rmax orless can be obtained by applying an ac high-frequency voltage of 7 MHzto 30 MHz between the electrode and the workpiece.

FIG. 10 is a block diagram illustrating an example of a conventionalpower supply unit for electrical discharge machining using an achigh-frequency power supply. In the drawing, reference numeral 1 denotesan electrode; 2, a workpiece; 3, a dc power supply; and 4, ahigh-frequency oscillation and amplification circuit. A fixed voltage orelectric power is inputted to the high-frequency oscillation andamplification circuit 4 from the dc power supply 3 through an externalcommand, which in turn generates an ac high-frequency voltage, and byapplying the ac high-frequency voltage Vg between the electrode 1 andthe work piece 2, thereby effecting electrical discharge machining ofthe workpiece 2 by electric discharge energy.

FIG. 11 is a diagram illustrating an example of a bath voltage waveformat the time of no-load in a case where an ac high-frequency voltage isapplied between the electrode and the workpiece in the conventionalpower supply unit for electrical discharge machining using the achigh-frequency power supply. By continuously supplying the achigh-frequency voltage between the electrode and the workpiece at afrequency of, e.g., 1 MHz or higher, it is possible to obtain a machinedsurface whose surface roughness is very smooth.

The above-described conventional power supply unit for electricaldischarge machining using the ac high-frequency power supply has a largeadvantage in that the surface roughness of the machined surface of theworkpiece becomes very smooth. However, it has become known that in acase where this power supply unit for electrical discharge machining isused in wire electrical discharge machining, there are a number ofproblems in order to meet exacting requirements in the recent market.The problems of the power supply unit for wire electrical dischargemachining using the ac high-frequency power supply are shown below.

(1) Straightness Accuracy Declines.

With the power supply unit for wire electrical discharge machining usingthe ac high-frequency power supply, since the voltage remains appliedbetween the electrode and the workpiece, an attracting force based onelectrostatic force acts between the wire electrode and the workpiece,so that a so-called “barrel shape” is formed in which a central portionof the workpiece is machined by a greater degree. Hence, there is aproblem in that the straightness accuracy declines.

(2) Streaks Occur in the Machined Surface.

Since the wire electrical discharge machining using the achigh-frequency power supply is the machining of an area where thesurface roughness is fine, even if there is the slightest vibration ofthe wire electrode, its effect imparted to the quality of the machinedsurface becomes noticeable. Accordingly, there is a problem in thatstreaks occur on the machined surface of the workpiece due to thevibration of the wire electrode occurring since the reaction force basedon the electric discharge and the attracting force based on theelectrostatic force acting between the wire electrode and the workpieceare not fixed. In addition, such streaks on the machined surface can beconfirmed by visual observation as well.

As for the machined surface of the workpiece based on the power supplyunit for wire electrical discharge machining using the ac high-frequencypower supply, it is not rare that a large difference occurs between thesurface roughness in the vertical direction (direction parallel to thewire electrode during machining) and the surface roughness in thehorizontal direction (direction perpendicular to the wire electrodeduring machining). For example, there are cases where the surfaceroughness in the horizontal direction is aggravated by 30 to 40 percentor thereabouts in comparison with the surface roughness in the verticaldirection. FIGS. 12A and 12B shows an example of the surface roughnessof the machined surface of the workpiece in a case where a 20 mm-thicksteel is machined with a 0.2 mm-diameter brass wire electrode by meansof the conventional power supply unit for wire electrical dischargemachining using the ac high-frequency power supply. FIG. 12A shows thecurve of surface roughness in the vertical direction, while FIG. 12Bshows the curve of surface roughness in the horizontal direction. In thecase of FIGS. 12A and 12B, the surface roughness in the horizontaldirection is 1.82 μm Rmax, and the surface roughness in the verticaldirection is 1.29 μm Rmax, so that it can be seen that the surfaceroughness in the horizontal direction is about 40% coarser than thesurface roughness in the vertical direction. The difference in surfaceroughness based on the direction is related to the streaks occurring onthe machined surface of the workpiece.

(3) The Machined Surface Roughness Declines.

With the power supply unit for wire electrical discharge machining usingthe ac high-frequency power supply, although the polarity of the voltagealternates, the voltage constantly remains applied, so that a phenomenonis noted in which the discharge continues for a long cycle(corresponding to several cycles to several dozen cycles). Hence, thereis a problem in that only the surface roughness which is several timescoarser than the surface roughness supposed to be obtained by thedischarge of one pulse (half wave of an alternating current) of an achigh frequency can be obtained. FIG. 13 shows an example of a bathvoltage waveform at the time of the execution of machining by theconventional power supply unit for electrical discharge machining usingthe ac high-frequency power supply with a power supply frequency of13.55 MHz. The presence or absence of the discharge can be primarilydetermined since the peak value of the ac high-frequency applied voltagedrops to a predetermined voltage or below. From the voltage waveformshown in FIG. 13, the phenomenon is observed in which the dischargetakes place continuously over several dozen cycles of the achigh-frequency applied voltage. In rough machining, it is known thatwhen the phenomenon in which the discharge occurs immediately after theapplication of the voltage has taken place, the discharge frequentlyoccurs in the same location. Also in electrical discharge machiningbased on the power supply unit for wire electrical discharge machiningusing the ac high-frequency power supply such as the one shown in FIG.13, it is estimated that discharge concentrations occur, and it isconceivable that the machined surface roughness declines due to thedischarge concentrations.

As applications of wire electrical discharge machining, applications forwhich extremely high precision and very smooth surface roughness arerequired are increasing in the semiconductor industry and otherindustries. For example, in the machining of such as dies for ICleadframes, there are such exacting requirements as the shape accuracybeing 1 μm or less and the surface roughness being 0.5 μm Rmax. To meetsuch exacting requirements, it has been a pressing task to overcome theabove-described problems.

DISCLOSURE OF THE INVENTION

The present invention has been devised to overcome the above-describedproblem, and its object is to obtain a power supply unit for wireelectrical discharge machining and a method of wire electrical dischargemachining which make it possible to obtain a high-precision andhigh-quality machined surface of the workpiece.

The power supply unit for wire electrical discharge machining inaccordance with the invention is a power supply unit for wire electricaldischarge machining using an ac high-frequency power supply for applyingan ac high-frequency voltage between a wire electrode and a workpiece,characterized by comprising: ac-high-frequency-voltage intermittentlysupplying means which effects the application of the ac high-frequencyvoltage between the wire electrode and the workpiece and a pausethereof.

In addition, in the power supply unit for wire electrical dischargemachining in accordance with the invention, a time of application of theac high-frequency voltage by the ac-high-frequency-voltageintermittently supplying means is set to a predetermined time which iscapable of interrupting the continuation of discharge.

In addition, in the power supply unit for wire electrical dischargemachining in accordance with the invention, a time of application of theac high-frequency voltage by the ac-high-frequency-voltageintermittently supplying means is set to approximately 1 μs or less.

In addition, in the power supply unit for wire electrical dischargemachining in accordance with the invention, a time of application of theac high-frequency voltage by the ac-high-frequency-voltageintermittently supplying means is set to not more than about 10 cyclesof the ac high-frequency voltage.

In addition, the power supply unit for wire electrical dischargemachining in accordance with the invention comprises: controlling meansin which a time of application of the ac high-frequency voltage by theac-high-frequency-voltage intermittently supplying means and a pausetime thereof as well as machining characteristics based on a peak valueand a frequency of the ac high-frequency voltage and the like are storedin advance, and stored values of the time of application of the achigh-frequency voltage and the pause time are invoked in accordance withrequired specifications and machining conditions so as to control theac-high-frequency-voltage intermittently supplying means.

In addition, the power supply unit for wire electrical dischargemachining in accordance with the invention comprises:ac-high-frequency-voltage variably supplying means which supplies thepower to the electrode gap by making smaller at a predetermined ratio anabsolute value of the ac high-frequency voltage in a secondpredetermined time than an absolute value of the ac high-frequencyvoltage in a first predetermined time, and which effects such variablevoltage supply repeatedly.

In addition, in the power supply unit for wire electrical dischargemachining in accordance with the invention, the first time ofapplication of the ac high-frequency voltage by theac-high-frequency-voltage variably supplying means is set to apredetermined time which is capable of interrupting the continuation ofdischarge.

In addition, in the power supply unit for wire electrical dischargemachining in accordance with the invention, the first time ofapplication of the ac high-frequency voltage by theac-high-frequency-voltage variably supplying means is set toapproximately 1 μs or less.

In addition, in the power supply unit for wire electrical dischargemachining in accordance with the invention, the first time ofapplication of the ac high-frequency voltage by theac-high-frequency-voltage variably supplying means is set to not morethan about 10 cycles of the ac high-frequency voltage.

In addition, the power supply unit for wire electrical dischargemachining in accordance with the invention comprises: controlling meansin which the first predetermined time and the second predetermined timeas well as machining characteristics based on a peak value and afrequency of the ac high-frequency voltage and the like are stored inadvance, and stored values of the first predetermined time and thesecond predetermined time are invoked in accordance with requiredspecifications and machining conditions so as to control theac-high-frequency-voltage variably supplying means.

In addition, the method of wire electrical discharge machining inaccordance with the invention is a method of wire electrical dischargemachining for machining a workpiece by supplying an ac high-frequencyvoltage between a wire electrode and the workpiece, characterized inthat the application of the ac high-frequency voltage between the wireelectrode and the workpiece is effected intermittently.

In addition, in the method of wire electrical discharge machining inaccordance with the invention, a time of application of the achigh-frequency voltage and a pause time thereof are made variable inaccordance with required specifications and machining conditions.

In addition, the method of wire electrical discharge machining inaccordance with the invention is a method of wire electrical dischargemachining for machining a workpiece by supplying an ac high-frequencyvoltage between a wire electrode and the workpiece, characterized inthat the ac high-frequency voltage is applied between the wire electrodeand the workpiece by making smaller at a predetermined ratio an absolutevalue of the ac high-frequency voltage in a second predetermined timethan an absolute value of the ac high-frequency voltage in a firstpredetermined time, and such variable voltage supply is effectedrepeatedly.

In addition, in the method of wire electrical discharge machining inaccordance with the invention, the first predetermined time and thesecond predetermined time are made variable in accordance with requiredspecifications and machining conditions.

Since the power supply unit for wire electrical discharge machining andthe method of wire electrical discharge machining in accordance with theinvention are arranged as described above, it is possible to improve thestraightness accuracy of the machined surface of the workpiece. Inaddition, it is possible to eliminate streaks formed on the machinedsurface of the workpiece. Further, it is possible to make the surfaceroughness of the machined surface of the workpiece small. Furthermore,it is possible to control the decline in machining productivity whilesecuring the desired shape accuracy and surface roughness of theworkpiece.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the power supply unit for wire electricaldischarge machining in accordance with a first embodiment of theinvention;

FIGS. 2A and 2B are conceptual diagrams for explaining the voltagewaveform of a pulse power supply and a bath voltage waveform in thepower supply unit for wire electrical discharge machining in accordancewith the first embodiment of the invention;

FIG. 3 is a diagram illustrating an example of the bath voltage waveformin a case where an intermittent ac high-frequency voltage is applied bythe power supply unit for wire electrical discharge machining inaccordance with the first embodiment of the invention;

FIGS. 4A and 4B are diagrams illustrating the results of measurement ofthe shape curve, straightness accuracy, and surface roughness of amachined surface of the workpiece;

FIG. 5 is a diagram illustrating the change of the surface roughness inthe vertical direction of the machined surface of the workpiece based ona predetermined time T1 of application of the ac high-frequency voltage;

FIG. 6 is a circuit diagram of a power supply unit for wire electricaldischarge machining in accordance with a second embodiment of theinvention;

FIGS. 7A and 7B are conceptual diagrams for explaining input signalsFETs and a bath voltage waveform in the power supply unit for wireelectrical discharge machining in accordance with the second embodimentof the invention;

FIG. 8 is a circuit diagram of the power supply unit for wire electricaldischarge machining in accordance with a third embodiment of theinvention;

FIGS. 9A and 9B are conceptual diagrams for explaining the input signalsFETs and the bath voltage waveform in the power supply unit for wireelectrical discharge machining in accordance with the third embodimentof the invention;

FIG. 10 is a block diagram illustrating an example of a conventionalpower supply unit for electrical discharge machining;

FIG. 11 is a diagram illustrating an example of a no-load voltagewaveform at a time when an ac high-frequency voltage is applied betweenthe electrode and the workpiece in the conventional power supply unitfor electrical discharge machining using the ac high-frequency powersupply;

FIGS. 12A and 12B are diagrams illustrating an example of the surfaceroughness of the machined surface of the workpiece in the conventionalpower supply unit for electrical discharge machining using the achigh-frequency power supply; and

FIG. 13 is a diagram illustrating an example of a bath voltage waveformat the time of the execution of machining by the conventional powersupply unit for electrical discharge machining using the achigh-frequency power supply.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment

FIG. 1 is a block diagram of the power supply unit for wire electricaldischarge machining in accordance with a first embodiment of theinvention. In the drawing, reference numeral 1 a denotes a wireelectrode; 2, a workpiece; 4, a high-frequency oscillation andamplification circuit; and 5, a pulse power supply. As an intermittentvoltage V is inputted from the pulse power supply 5 to thehigh-frequency oscillation and amplification circuit 4, an achigh-frequency voltage with a voltage Vg1 is applied between the wireelectrode 1 a and the workpiece 2.

FIGS. 2A and 2B are conceptual diagrams for explaining the voltagewaveform V of the pulse power supply 5 and the bath voltage waveform Vg1in the power supply unit for wire electrical discharge machining inaccordance with the first embodiment of the invention. An achigh-frequency voltage is applied intermittently between the electrodeand the workpiece in synchronism with a predetermined time T1 duringwhich a pulse voltage V outputted from the pulse power supply 5 isapplied and a predetermined time T2 during which the pulse power supply5 is not applied.

The pulse power supply 5 corresponds to an ac-high-frequency-voltageintermittently supplying means which effects the application of the achigh-frequency voltage between the electrode and the workpiece(predetermined time T1 ) and a pause (predetermined time T2 ) thereof.

FIG. 3 shows an example of the bath voltage waveform Vg1 in a case wherethe ac high-frequency voltage with a frequency of 13.55 MHz isintermittently applied with the predetermined time T1=1.7 μs and thepredetermined time T2=2 μs. The fact that the amplitude of the bathvoltage waveform Vg1 is not fixed as compared with the conceptualdiagram shown in FIG. 2A and 2B is due to such as the effect of timeconstants of the rise and fall.

By applying the ac high-frequency voltage intermittently in theabove-described manner, the electrostatic force occurring in theelectrode gap during the predetermined time T2 when the achigh-frequency voltage is not applied can be reduced, so that it becomespossible to suppress the vibration of the wire electrode 1 a, therebymaking it possible to realize high-precision machining.

FIGS. 4A and 4B show the results of measurement of the shape curve,straightness accuracy, and surface roughness of the machined surface ofthe workpiece. FIG. 4A shows the shape curve and the like in a casewhere a 13.55 MHz ac high-frequency voltage was applied continuously,while FIG. 4B shows the shape curve and the like in a case (T1=1.7 μs,T2=2 μs) where the 13.55 MHz ac high-frequency voltage was appliedintermittently. In addition, both of FIGS. 4A and 4B show the results ofmeasurement in a case where the workpiece was a 20 mm-thick steel andthe wire electrode was 0.2 mm-diameter brass.

Looking at the shape curve in the case where the ac high-frequencyvoltage was applied continuously in FIG. 4A, the so-called “barrelshape” was formed in which the central portion of the workpiece wassubstantially concaved, and the straightness accuracy was 2.39 μm andwas poor; however, the straightness accuracy in the case where the achigh-frequency voltage was applied intermittently in FIG. 4B was 0.19μm. Thus it was possible to confirm that the straightness accuracy canbe substantially improved by the intermittent application of the achigh-frequency voltage.

In addition, it was found that in the case where the ac high-frequencyvoltage was applied intermittently in FIG. 4B, as compared with the casewhere the ac high-frequency voltage was applied continuously in FIG. 4A,the difference between the surface roughness in the horizontal directionand the surface roughness in the vertical direction of the machinedsurface of the workpiece was small. Also, it was confirmed that in thecase where the ac high-frequency voltage was applied intermittently inFIG. 4B, streaks on the machined surface of the workpiece occurring dueto the vibration of the wire electrode disappear.

However, it can be understood that the surface roughness of the machinedsurface of the workpiece cannot be improved substantially under thecondition of intermittent application of the ac high-frequency voltage.

This is conceivably attributable to the fact that since thepredetermined time T1 of application of the ac high-frequency voltage islong, the intermittent application of the ac high-frequency voltage isnot much different from the continuous application of the achigh-frequency voltage in terms of the continuation of the discharge.Accordingly, when discharges were effected by making short thepredetermined time T1 of application of the ac high-frequency voltage tointerrupt the continuation of the discharge by the ac high frequency, itwas found that the surface roughness of the machined surface of theworkpiece can be made very small if the predetermined time T1 ofapplication of the ac high-frequency voltage is set to not more than apredetermined value.

FIG. 5 shows the change of the surface roughness in the verticaldirection of the machined surface of the workpiece based on thepredetermined time T1 of application of the ac high-frequency voltage,and reveals that the surface roughness of the machined surface of theworkpiece becomes very small and high-quality surface qualities can beobtained if the predetermined time T1 of application of the achigh-frequency voltage is set to not more than approximately 1 μs orthereabouts (corresponding to about 10 to 10 odd cycles of the achigh-frequency voltage). For example, in the case where thepredetermined time T1 of application of the ac high-frequency voltagewas 0.6 μs, the surface roughness in the vertical direction of themachined surface of the workpiece was 0.76 μm Rmax (the surfaceroughness in the horizontal direction: 0.89 μm Rmax), so that it can beunderstood that the surface roughness can be improved substantially incomparison with the results of measurement of the surface roughness ofthe machined surface of the workpiece shown in FIGS. 4A and 4B.

As described above, although it was found that the machiningcharacteristics such as the straightness accuracy, the surfaceroughness, and the like of the workpiece can be improved substantiallyby the intermittent application of the ac high-frequency voltage andselecting the predetermined time T1 of application of the achigh-frequency voltage, the machining efficiency declines in comparisonwith the case of the continuous application of the ac high-frequencyvoltage. Accordingly, if the machining characteristics based on thepredetermined times T1 and T2 as well as the peak value and thefrequency of the ac high-frequency voltage and the like are determinedin advance by experiments and are stored in an unillustrated controllingmeans, and if the predetermined times T1 and T2 are set by thecontrolling means in accordance with required specifications andmachining conditions, it is possible to control the decline in machiningproductivity while securing the desired shape accuracy and surfaceroughness.

For example, if the thickness of the workpiece becomes large, the effectdue to the electrostatic force becomes large, so that if theelectrostatic force is made small by lowering the proportion of thepredetermined time T1 by enlarging the predetermined time T2, it ispossible to improve the straightness accuracy. Meanwhile, in a casewhere the amount of machining becomes large, by making the predeterminedtime T1 large by making the predetermined time T2 small, it is possibleto realize stable machining by increasing the machining capability.

To apply the above-described ac high-frequency voltage between theelectrode and the workpiece, there are various methods in addition tothe use of the pulse power supply 5, such as the one shown in FIG. 1, asthe ac-high-frequency-voltage intermittently supplying means. Forexample, a dc power supply may be used instead of the pulse power supply5, and the voltage supplied to the high-frequency oscillation andamplification circuit 4 may be turned on and off by a switching device,or an arrangement may be provided such that a continuous achigh-frequency voltage is generated beforehand as the power supply, andthe electrode gap is forcibly shortcircuited periodically.

In addition, the invention disclosed in, for example, JP-A-59-232726 isknown as being related to the above-described concept of interposingpauses in the ac high-frequency power supply. In that invention, thereare provided a first power supply for applying an ac high-frequencyvoltage and a second power supply for allowing a pulse current of apredetermined width to flow from the point of time of starting thedischarge, and electrical discharge machining is effected by the pulsecurrent when a discharge has occurred on application of the achigh-frequency voltage between the electrode and the workpiece, and thatinvention concerns a power supply of a type in which the achigh-frequency voltage is used to induce the discharge, and a dc currentis applied after detecting the discharge. Accordingly, the machiningspeed and the electrode wear are determined by the aforementioned pulsecurrent, and the technical concept utterly differs from the presentinvention.

Second Embodiment

FIG. 6 is a circuit diagram of a power supply unit for wire electricaldischarge machining in accordance with a second embodiment of theinvention. In the drawing, reference numeral 1 a denotes the wireelectrode; 2, the workpiece; 3, a dc:power supply; 4, the high-frequencyoscillation and amplification circuit; and 6, a resistance device.Numerals 7 a and 7 b denote resistors; 8 a and 8 b, diodes; and 9 a and9 b, FETs. As a predetermined voltage is inputted from the dc powersupply 3 to the high-frequency oscillation and amplification circuit 4,an ac high-frequency voltage with a voltage Vg2 is applied between thewire electrode 1 a and the workpiece 2. The FETs 9 a and 9 b in theresistance device 6 connected in parallel to the electrode gap arearranged to be capable of being independently driven by external signalsS1 and S2. In addition, the diode 8 a is connected in series to theresistor 7 a in a direction in which the current flows when the polarityof the wire electrode 1 a is minus and the polarity of the workpiece 2is plus (straight polarity), whereas the diode 8 b is connected inseries to the resistor 7 b in a direction in which the current flowswhen the polarity of the wire electrode 1 a is plus and the polarity ofthe workpiece 2 is minus (reverse polarity)

FIGS. 7A and 7B are conceptual diagrams for explaining the respectiveinput signals S1 and S2 of the FET 9 a and the FET 9 b and a bathvoltage waveform Vg2 in the power supply unit for wire electricaldischarge machining in accordance with the second embodiment of theinvention. In a case where the on and off operation of the FET 9 a andthe FET 9 b is effected in synchronism as in FIG. 7A, the absolute valueof the ac high-frequency voltage Vg2 applied between the electrode andthe workpiece becomes smaller in the case of a predetermined time T3 (asecond predetermined time) when the external signals S1 and S2 are onthan in the case of a predetermined time T4 (a first predetermined time)when the external signals S1 and S2 are off, as shown in FIG. 7B.

The resistance device 6 in FIG. 6 corresponds to anac-high-frequency-voltage variably supplying means which effects thesupply to the electrode gap by making smaller at a predetermined ratiothe absolute value of the ac high-frequency voltage for the secondpredetermined time than for the first predetermined time, and whicheffects such variable voltage supply repeatedly.

By applying such an ac high-frequency voltage between the electrode andthe workpiece, it is possible to reduce the electrostatic forceoccurring between the wire electrode 1 aand the workpiece 2 during thepredetermined time T3, so that it becomes possible to suppress thevibration of the wire electrode 1 a and realize machining with greaterprecision.

Third Embodiment

FIG. 8 is a circuit diagram of the power supply unit for wire electricaldischarge machining in accordance with a third embodiment of theinvention, and the same reference numerals as those in FIG. 6 inaccordance with the second embodiment denote identical or correspondingportions. In FIG. 8, reference numeral 5 denotes the pulse power supply;10, a resistance device; 11, a resistor. Numerals 12 a and 12 b denotediodes; and 13 a and 13 b, FETs. As a predetermined voltage is inputtedfrom the dc power supply 3 to the high-frequency oscillation andamplification circuit 4, an ac high-frequency voltage Vg3 is appliedbetween the wire electrode 1 a and the workpiece 2. The FETs 13 a and 13b in the resistance device 10 connected in parallel to the electrode gapare arranged to be capable of being simultaneously driven by the pulsepower supply 5. At the time of the reverse polarity, the current flowsacross the resistor 11 through the FET 13 a and the diode 12 b of theFET 13 b, while at the time of the straight polarity, the current flowsacross the resistor 11 through the diode 12 a of the FET 13 a and theFET 13 b.

FIGS. 9A and 9B are conceptual diagrams for explaining respective inputsignals S3 and S4 of the FET 13 a and the FET 13 b and the bath voltagewaveform Vg3 in the power supply unit for wire electrical dischargemachining in accordance with the third embodiment of the invention. Asthe FET 13 a and the FET 13 b are turned on and off in synchronism as inFIG. 9A, as shown in FIG. 9B, it is possible to obtain a bath voltagewaveform similar to that in FIG. 7B in accordance with the secondembodiment, and an advantage similar to that of the second embodiment isoffered.

In addition, in this case, the resistance device 10 in FIG. 8corresponds to the ac-high-frequency-voltage variably supplying meanswhich effects the supply to the electrode gap by making smaller at apredetermined ratio the absolute value of the ac high-frequency voltagefor the second predetermined time than for the first predetermined time,and which effects such variable voltage supply repeatedly.

INDUSTRIAL APPLICABILITY

As described above, the power supply unit for wire electrical dischargemachining and the method of wire electrical discharge machining inaccordance with the invention are particularly suitable for use inhigh-precision and high-quality wire electrical discharge machining.

1. A power supply unit for wire electrical discharge machiningcomprising an ac high-frequency power supply operable to intermittentlysupply an ac high-frequency voltage between a wire electrode and aworkpiece to machine the workpiece, wherein a period of time duringwhich the ac high-frequency voltage is applied by said ac high-frequencypower supply is set to a predetermined time that is capable ofinterrupting the continuation of discharge.
 2. The power supply unit forwire electrical discharge machining according to claim 1, furthercomprising: a controller in which a period of time during which the achigh-frequency voltage is applied by said ac high-frequency power supplyand a pause time thereof as well as machining characteristics based on apeak value and a frequency of the ac high-frequency voltage and the likeare stored in advance, and the stored values of the period of timeduring which the ac high-frequency voltage is applied and the pause timeare invoked in accordance with one or more required specifications formachining the workpiece and machining conditions so as to control saidac high-frequency power supply.
 3. The power supply unit for wireelectrical discharge machining according to claim 1, wherein a time ofapplication of the ac high-frequency voltage by said ac high-frequencypower supply is set to approximately 1 μs or less.
 4. The power supplyunit for wire electrical discharge machining according to claim 1,wherein a period of time during which the ac high-frequency voltage isapplied by said ac high-frequency power supply is set to not more thanabout 10 cycles of the ac high-frequency voltage.
 5. A power supply unitfor wire electrical discharge machining comprising an ac high-frequencypower supply for applying an ac high-frequency voltage between a wireelectrode and a workpiece by making smaller at a predetermined ratio anabsolute value of the ac high-frequency voltage in a secondpredetermined time than an absolute value of the ac high-frequencyvoltage in a first predetermined time, and which repeatedly supplies thevariable voltage, wherein the first period of time during which the achigh-frequency voltage is applied by said ac high-frequency power supplyis set to a predetermined time which is capable of interrupting thecontinuation of discharge.
 6. The power supply unit for wire electricaldischarge machining according to claim 5, wherein the first period oftime during which the ac high-frequency voltage is applied by said achigh-frequency power supply is set to not more than about 10 cycles ofthe ac high-frequency voltage.
 7. The power supply unit for wireelectrical discharge machining according to claim 5, comprising: acontroller in which the first predetermined period of time and thesecond predetermined time as well as machining characteristics based ona peak value and a frequency of the ac high-frequency voltage and thelike are stored in advance, and the stored values of the firstpredetermined time and the second predetermined time are invoked inaccordance with one or more required specifications for machining theworkpiece and machining conditions so as to control said achigh-frequency power supply.
 8. The power supply unit for wireelectrical discharge machining according to claim 5, wherein the firstperiod of time during which the ac high-frequency voltage is applied bysaid ac high-frequency power supply is set to approximately 1 μs orless.
 9. A method of wire electrical discharge machining for machining aworkpiece by supplying an ac high-frequency voltage between a wireelectrode and the workpiece, wherein the ac high-frequency voltage isapplied between the electrode and the workpiece by making smaller at apredetermined ratio an absolute value of the ac high-frequency voltagein a second predetermined time than an absolute value of the achigh-frequency voltage in a first predetermined time, and the variablevoltage is repeatedly applied wherein the first period of time duringwhich the ac high-frequency voltage is applied by said ac high-frequencypower supply is set to a predetermined time which is capable ofinterrupting the continuation of discharge.
 10. The method of wireelectrical discharge machining according to claim 9, wherein the firstpredetermined time and the second predetermined time are set inaccordance with one or more required specifications for machining theworkpiece and machining conditions.
 11. A method of wire electricaldischarge machining for machining a workpiece by supplying an achigh-frequency voltage between a wire electrode and the workpiece,wherein: the ac high-frequency voltage is intermittently applied betweenthe wire electrode and the workpiece to machine the workpiece, and aperiod of time during which the ac high-frequency voltage is applied isset to a predetermined time that is capable of interrupting thecontinuation of discharge.
 12. The method of wire electrical dischargemachining according to claim 11, wherein the period of time during whichthe ac high-frequency voltage is applied and a pause time thereof areset in accordance with one or more required specifications for machiningthe workpiece and machining conditions.